Blender Git Loki

Sculpt: multithread GPU draw buffer filling for workbench

This improves performance of some sculpt tools, particularly those that modify
many vertices like filter and mask tools, or use brushes with large radius.

For mask expand it can make updates up to 2x faster on heavy meshes, but for
most tools it's more on the order of 1-1.1x. There are bigger bottlenecks to
solve, like normal updates.

Ref T70295

Differential Revision: https://developer.blender.org/D5926